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<H1 id="MPI_Intercomm_create">MPI_Intercomm_create</H1>
Creates an intercommuncator from two intracommunicators 
<H2>Synopsis</H2>
<PRE>
</PRE>
<PRE>
int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader,
                         MPI_Comm peer_comm, int remote_leader, int tag,
                         MPI_Comm *newintercomm)
</PRE>

<P>
<H2>Input Parameters</H2>
<DL>
<DT><B>local_comm </B> <DD> local intra-communicator (handle)

<DT><B>local_leader </B> <DD> rank of local group leader in local_comm (integer)

<DT><B>peer_comm </B> <DD> `peer communicator; significant only at the local_leader (handle)

<DT><B>remote_leader </B> <DD> rank of remote group leader in peer_comm; significant only at the local_leader (integer)

<DT><B>tag </B> <DD> tag (integer)
</DL>
<P>
<H2>Output Parameters</H2>
<DL>
<DT><B>newintercomm </B> <DD> new inter-communicator (handle)
</DL>
<P>
<H2>Notes</H2>
<tt>peer_comm</tt> is significant only for the process designated the
<tt>local_leader</tt> in the <tt>local_comm</tt>.
<P>
The MPI 1.1 Standard contains two mutually exclusive comments on the
input intercommunicators.  One says that their respective groups must be
disjoint; the other that the leaders can be the same process.  After
some discussion by the MPI Forum, it has been decided that the groups must
be disjoint.  Note that the <em>reason</em> given for this in the standard is
<em>not</em> the reason for this choice; rather, the <em>other</em> operations on
intercommunicators (like <tt>MPI_Intercomm_merge</tt>) do not make sense if the
groups are not disjoint.
<P>
<H2>Thread and Interrupt Safety</H2>
<P>
This routine is thread-safe.  This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks.  However, the routine is not interrupt safe.  Typically,
this is due to the use of memory allocation routines such as <tt>malloc
</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
<P>
<H2>Notes for Fortran</H2>
All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
an additional argument <tt>ierr</tt> at the end of the argument list.  <tt>ierr
</tt>is an integer and has the same meaning as the return value of the routine
in C.  In Fortran, MPI routines are subroutines, and are invoked with the
<tt>call</tt> statement.
<P>
All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
</tt>in Fortran.
<P>
<H2>Errors</H2>
<P>
All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) return an error value;
C routines as the value of the function and Fortran routines in the last
argument.  Before the value is returned, the current MPI error handler is
called.  By default, this error handler aborts the MPI job.  The error handler
may be changed with <tt>MPI_Comm_set_errhandler</tt> (for communicators),
<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
RMA windows).  The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
its use is deprecated.  The predefined error handler
<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
Note that MPI does <em>not</em> guarantee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.
<P>
<DL>
<DT><B>MPI_SUCCESS </B> <DD> No error; MPI routine completed successfully.
</DL>
<DL>
<DT><B>MPI_ERR_ARG </B> <DD> Invalid argument.  Some argument is invalid and is not
identified by a specific error class (e.g., <tt>MPI_ERR_RANK</tt>).
</DL>
<DL>
<DT><B>MPI_ERR_COMM </B> <DD> Invalid communicator.  A common error is to use a null
communicator in a call (not even allowed in <tt>MPI_Comm_rank</tt>).
</DL>
<DL>
<DT><B>MPI_ERR_TAG </B> <DD> Invalid tag argument.  Tags must be non-negative; tags
in a receive (<tt>MPI_Recv</tt>, <tt>MPI_Irecv</tt>, <tt>MPI_Sendrecv</tt>, etc.) may
also be <tt>MPI_ANY_TAG</tt>.  The largest tag value is available through the 
the attribute <tt>MPI_TAG_UB</tt>.
</DL>
<DL>
<DT><B>MPI_ERR_OTHER </B> <DD> Other error; use <tt>MPI_Error_string</tt> to get more information
about this error code. 
</DL>
<P>
<H2>See Also</H2>
 MPI_Intercomm_merge, MPI_Comm_free, MPI_Comm_remote_group, MPI_Comm_remote_size
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